High-temperature interface superconductivity between metallic and insulating cuprates

TL;DR

This study demonstrates that high-temperature superconductivity can be induced and enhanced at the interfaces of bilayers composed of insulating and metallic cuprates, with T_c exceeding 50 K under certain conditions.

Contribution

It reveals that engineering atomically perfect interfaces in cuprate bilayers can create and significantly boost high-temperature superconductivity, a novel approach in the field.

Findings

Superconductivity appears at the interface of La2CuO4 and La1.55Sr0.45CuO4 bilayers.

T_c reaches ~15 K or ~30 K depending on layering sequence.

Exposing bilayers to ozone raises T_c above 50 K, originating from a 1-2 unit cell thick interface layer.

Abstract

High-temperature superconductivity confined to nanometer-size interfaces has been a long standing goal because of potential applications^{1,2} and the opportunity to study quantum phenomena in reduced dimensions^{3,4}. However, this is a challenging target: in conventional metals the high electron density restricts interface effects such as carrier depletion/accumulation to a region much narrower than the coherence length, the scale necessary for superconductivity to occur. In contrast, in copper oxides the carrier density is low while the critical temperature (T_c) is high and the coherence length very short; so, this provides a breakthrough opportunity but at a price: the interface must be atomically perfect. Here we report on superconductivity in bilayers consisting of an insulator (La_2CuO_4) and a metal (La_{1.55}Sr_{0.45}CuO_{4}), neither of which is superconducting in isolation. However, in bilayers T_c is either ~15 K or ~30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm from the interface. If such a bilayer is exposed to ozone, T_c exceeds 50 K and this enhanced superconductivity is also shown to originate from the interface layer about 1-2 unit cell thick. Enhancement of T_c in bilayer systems was observed previously^5 but the essential role of the interface was not recognized at the time. Our results demonstrate that engineering artificial heterostructures provides a novel, unconventional way to fabricate stable, quasi two-dimensional high T_c phases and to significantly enhance superconducting properties in known or new superconductors.